1. Field of the Invention
The present invention relates to an image sensing apparatus and a method of controlling the image sensing apparatus.
2. Description of the Related Art
Some image sensing apparatuses such as digital cameras and digital video cameras have an operation mode in which still image shooting is done during movie shooting. In the image sensing apparatus, an image sensor such as a CCD sensor or a CMOS sensor includes a pixel array in which a plurality of pixels are arrayed in the row and column directions, and a read circuit for reading out a signal from the pixel array. These days, the number of pixels included in the pixel array tends to increase (megapixel array).
In the operation mode in which movie shooting is done to acquire a plurality of successive frame images, the image sensing apparatus performs an operation suited to movie shooting. More specifically, the image sensing apparatus drives the read circuit to read out signals from the pixel array at a pixel density decreased by addition, decimation, extraction, or the like, so as to read out signals in a readout period in which a frame rate suitable for movie shooting can be achieved.
In the operation mode in which still image shooting is done during movie shooting, the image sensing apparatus executes an operation which gives priority to still image shooting. More specifically, the image sensing apparatus drives the read circuit to read out signals in a readout period longer than the readout period in which a frame rate suitable for movie shooting can be achieved, so as to read out signals from the pixel array at a high pixel density suitable for still image shooting. Also, the image sensing apparatus controls each pixel of the pixel array so as to accumulate a signal at a shutter speed, that is, in an accumulation period suited to still image shooting. In still image shooting, the shutter speed, that is, accumulation period needs to be set shorter than an accumulation period suited to movie shooting (accumulation period in the operation mode in which movie shooting is done), in order to prevent blurring of an image upon the movement of an object.
In the image sensing apparatus, the operation mode sometimes switches between successive frames from the operation mode in which movie shooting is done to the operation mode in which still image shooting is done during movie shooting. In this case, the following two problems arise in the image sensing apparatus.
As described above, the readout period in the operation mode in which still image shooting is done during movie shooting is longer than that in the operation mode in which movie shooting is done. In the image sensing apparatus, a length of a readout period changes discontinuously upon switching between the operation modes, so a length of a frame period changes discontinuously. The continuity of evaluation values for motion detection of an object and the like may be lost (first problem). It may become difficult to standardize discontinuous evaluation values. If a predetermined control operation is performed using such evaluation values, the precision of the control operation may decrease.
For example, if the length of the frame period changes discontinuously, the continuity of evaluation values for motion detection of an object (moving amounts of an object) may be lost. If a control operation such as camera shake correction of the image sensing apparatus is executed using discontinuous moving amounts of an object, the precision of the control operation decreases.
As described above, the accumulation period of each pixel in the operation mode in which still image shooting is done during movie shooting is shorter than that of each pixel in the operation mode in which movie shooting is done. In the image sensing apparatus, a length of an accumulation period of each pixel changes discontinuously upon switching the operation mode, so the level of a signal read out from each pixel changes discontinuously. The continuity of evaluation values for photometry and the like may be lost (second problem). It may become difficult to standardize discontinuous evaluation values. If a predetermined control operation is performed using such evaluation values, the precision of the control operation may decrease.
For example, if the level of a signal read out from each pixel changes discontinuously, the continuity of evaluation values for photometry may be lost. If a control operation (AE control operation) such as exposure compensation of the image sensing apparatus is executed using discontinuous evaluation values for photometry, the precision of the control operation decreases.
A technique disclosed in Japanese Patent Laid-Open No. 09-023371 proposes to keep constant the level of a signal input to an A/D converter by dynamically adjusting the gain of an amplifier (AGC circuit) on the output stage of an image sensor upon detecting a change of the level of a signal read out from each pixel. According to this technique, when a length of a readout period changes discontinuously upon switching between the operation modes and a length of a frame period changes discontinuously, the continuity of evaluation values for motion detection of an object and the like may be lost. The technique disclosed in Japanese Patent Laid-Open No. 09-023371 cannot solve the first problem.
According to this technique, when a length of a accumulation period of each pixel changes discontinuously upon switching between the operation modes and the level of a signal read out from each pixel changes discontinuously, the level of a signal input to the A/D converter may not be able to be kept constant. Even if the gain of the amplifier on the output stage of the image sensor is dynamically adjusted, the adjusted gain may contain an error, failing to keep constant the level of a signal input to the A/D converter. As a result, the signal level changes discontinuously, and the continuity of evaluation values for photometry and the like may be lost. The technique disclosed in Japanese Patent Laid-Open No. 09-023371 cannot solve the second problem.
A technique disclosed in Japanese Patent Laid-Open No. 2005-277513 proposes to read out, from the first pixel group, a signal for generating an evaluation value, and a signal for an EVF image from the second pixel group in a pixel array in which a plurality of pixels are arrayed. Japanese Patent Laid-Open No. 2005-277513 does not disclose an operation of the image sensing apparatus upon switching between the operation modes. The technique disclosed in Japanese Patent Laid-Open No. 2005-277513 can solve neither the first problem nor the second problem.